The following are features of adaptive and beneficial training for masters swimmers.

Enjoy what you are doing. If it gets hard or provokes negative thoughts, ease off.

Small gains over a long time are better than large gains over a short time. Very gradual adaptation stays with you longer and reaches a higher ceiling than does short and fast exposure.

There is an optimal amount of distance that needs to be covered at a particular race pace in interval training. My "guestimate" is as follows:

For events 400 m and less, three times the racing distance is enough. Too much anaerobic work will precipitate overreaching and if continued, overtraining.

For events more than 400 m, two to two and a half times racing distance is enough.

Remember, the interval sets included in these interval-set distances are at intended race pace, not slower nor faster.
What if you complete sets easy when you are doing 2000 m for and intended 800 m race? Speed up the race pace equivalent, reduce the distance, and start building up to the maximum distance again.

Never continue when the work is perceived to be very hard, your pace has dropped off the target, and your technique has changed (it has become a "slog." Work done beyond adaptive work is counter-productive and negates the benefits of useful training. This is the tough part of training for individuals who have strong work ethic. Always remember part III of roux's Principle: "Excessive work is harmful."

Work intervals should be:

For 100 m repetition distance or less, one time period for work (race pace), half a time period for recovery (a one to a half work:recovery ratio).

For 200 m repetition distance or more, one time period for work, and two time periods for recovery (a one to two work:recovery ratio).

Keep the non-specific filler work of practice at or below the anaerobic threshold (the pace you would swim for a 3000 m time trial).

When you are not doing race specific work, the remainder of swimming should be building or maintaining aerobic base or recovery.

Some people would question why never get into the "agony" zone at practice. The "hurt-pain-agony" sequence that Jim Counsilman made famous in the 1960s was appropriate when swimmers, particularly US age-groupers, were not doing enough work. That no longer is true so the concept is no longer relevant. There just is no place in beneficial work for damaging fatigue states to be endured. There is no scientific evidence to support it although there is a growing volume evidence that will support the contention of moderate stimulation being the most beneficial (Roux's Principle Part II: "Moderate work is beneficial"). I rely heavily on German research to support this contention with backup from David Costill's laboratory at Ball State University. My on-going associations in Australia give practical verification to these principles.